The osteoclast plays a key role in the pathogenesis of osteoporosis and other metabolic bone diseases. While our understanding of the origin and regulation of the osteoclast has improved greatly in recent years, we are still almost completely ignorant of the fate of the osteoclast. As part of an investigation of the cellular mechanisms underlying glucocorticoid- induced osteoporosis, we have obtained evidence that the osteoclast exhibits apoptosis, or programmed cell death, a distinctive and deliberate type of cell death that differs fundamentally from necrosis in both its nature and biological significance. Prompted by this observation, this revised application is focused on the hypothesis that apoptosis plays a physiological role in determining osteoclast life span and, thereby, in the regulation of bone resorption. The significance of this hypothesis lies in the fact that it has the potential to provide a new conceptual framework for understanding important aspects of skeletal biology that are currently poorly understood, for example, the mode of action of glucocorticoids and estrogens on bone. The specific aims of this proposal are: 1) to test the hypothesis that in the presence of glucocorticoids, osteoclasts exhibit apoptosis in vitro, to elucidate the intracellular mechanisms underlying apoptosis in osteoclasts, and to probe for molecular markers of apoptosis in dying osteoclasts; 2) to examine how effects of glucocorticoids on osteoclast survival and activity are modified by the presence of osteoblasts and to determine the effects of glucocorticoids on osteoclast recruitment; 3) to determine whether mammalian and avian osteoclasts express glucocorticoid receptors; and 4) to test the hypothesis that osteoclasts exhibit apoptosis in vivo. These aims will be pursued using a combination of techniques, including the disaggregated osteoclast resorption assay, transmission electron microscopy of the osteoclasts, in situ hybridization, immunocytochemistry, and bone histomorphometry.